1. Field of Invention
This invention relates generally to instruments for examining and treating the eye and specifically to a scanning laser ophthalmoscope with external laser sources for the purpose of retinal photocoagulation or the measurement of wavefront aberrations of the eye optics.
2. Description of Prior Art
The ophthalmoscope is well known as an important device for examining the eye, and in particular the retina. As a result of great interest in preserving eyesight, ophthalmoscopes of various constructions have been built. The latest version of the ophthalmoscope, a scanning laser ophthalmoscope, is particularly appealing because of its unique capability of combining the infra-red and angiographical imaging of the retina with psychophysical procedures such as the study of visual fixation, visual acuity measurements, and microperimetry. Only with the scanning laser ophthalmoscope, a unique, precise correlation between retinal anatomy and function can be established. This retinal function mapping is now known to be very helpful to the surgeon when delivering therapeutic laser applications to the retina. Until now however, these therapeutic laser applications have been delivered to the retina with an instrument other than the scanning laser ophthalmoscope. The use of different instruments renders the comparison of images and the interpretation of psychophysical testing more difficult.
U.S. Pat. No. 4,213,678, issued Sep. 29, 1980 to Pomerantzeff et al, discloses a co-pupillary scanning laser ophthalmoscope for the purpose of diagnosing and treating retinal disease using two different intensity levels of the scanning laser beam. One intensity range can be used for monochromatic imaging and angiography while a much higher level of the same laser beam or a different colinear scanning laser beam is used for retinal photocoagulation. This novel approach however is not ideal because of the technical difficulties in implementing safety controls for such scanning therapeutic laser beam, the difficulty in modulating the scanning laser beam over a range from non-coagulating to coagulating energies at video bandwiths, and the non-thermal complications of high intensity pulsed laser beams in the nanosecond domain.
In the prior art, an ophthalmoscope, for example the slitlamp or biomicroscope, is optically combined with a non-scanning therapeutic laser source for the purpose of retinal photocoagulation. In this modality, a contactglass is usually placed on the cornea to be able to view the retina with the instrument, and a mirror is used for reflecting the therapeutic laser beam onto the desired retinal location through a small part of the pupillary area. Importantly, the retina is illuminated and observed through different parts of the pupillary area to avoid reflexes, i.e. Gullstrand's principle of ophthalmoscopy. This optical arrangement makes the art of precise focussing of a small therapeutic laser beam on specific retinal levels more difficult in the presence of wavefront aberrations.
Furthermore, photocoagulating ophthalmoscopes have also been limited when such consistent small and localized laser applications in the retina are desired because the anatomical changes are difficult if not impossible to visualize during treatment in the presence of photocoagulating light. The critical endpoint of the laser application is therefore often exceeded. The surgeon, upon recognizing the minimal anatomical changes on the retina, is handicapped by a substantial human reaction time delay before he can interrupt the therapeutic laser. During this delay the laser continues to deliver energy to the retina and changes in the subject's fixation occur. Since the reaction time of the surgeon may exceed 200 ms, a 100 ms laser application can easily be wrongly targeted on the retina in the case of misalignment.
Also, it is difficult to avoid re-application of therapeutic laser to the same location or too close to another laser application because no reference to previous applications is available on the retinal image and the applications themselves are usually not visible some time after the initial treatment.